The present invention relates to an image reading apparatus and, more particularly, to an image reading apparatus which can be suitably applied as an original image input unit of an image processing apparatus such as a scanner, a copying machine, or a facsimile apparatus.
Conventionally, an image processing apparatus such as a scanner, a copying machine, or a facsimile apparatus widely employs a so-called line sensor (line image sensor) as an image reading unit for reading an original image. This line sensor has a plurality of photoelectric conversion elements such as CCDs (Charge Coupled Devices) so as to convert reflected light obtained by irradiating an original image with light from a light source into an electrical signal.
Generally, in an image reading unit having such a line sensor, to correct variations in sensitivity among photoelectric conversion elements arrayed in a line on the line sensor, so-called shading correction processing is performed to optimize the gain in correspondence with the individual photoelectric conversion elements and the lamp characteristics of a light source.
A general shading correction operation will be briefly described with reference to an image processing apparatus or a facsimile apparatus shown in FIG. 2. To generate shading correction data, a control unit (not shown) of the image processing apparatus reads a predetermined white reference plate 202 located near an original table 201 through a line sensor 105 and generates shading correction data on the basis of an image signal output from the line sensor 105 upon reading. The shading correction data generated with this procedure is stored in the memory of the control unit and directly used when an original image is actually read to generate the input image data of the original image.
As the image reading unit of such an image processing apparatus, a unit incorporating a so-called ADF (Automatic Document Feeder) for automatically feeding a plurality of originals to the original table one by one is also popularly used. In such a image reading unit, just like a conventional image reading unit like a unit employed in a general facsimile apparatus, when a plurality of originals are to be read, the line sensor is fixed at a predetermined image read position, and the originals are fed to that position one by one. In this image reading unit, the charge storage time of photoelectric conversion elements, which is used to read the originals fed by the ADF, is the same as the charge storage time of the photoelectric conversion elements, which is used to read the image of one original directly mounted on the original table by the user while moving the line sensor. In addition, when an original image is to be read, shading correction data which has been generated in advance is generally commonly used independently of use or nonuse of the ADF.
Hence, such an image reading unit need be designed such that the density of an input image obtained by reading a reference original image using the ADF (read while fixing the line sensor at a predetermined position) equals the density of an input image obtained by reading the reference original image mounted on the original table (read while moving the line sensor).
However, it is generally difficult to design an image reading unit while equalizing the input image densities. In many cases, a large optical difference is generated between reading an original using the ADF and reading an original placed on the original table in advance by, e.g., a difference in distance between the SELFOC lens of the line sensor and the original mount surface or a difference in diffused reflection due to the structure around the reading mechanism in the unit. To correct this difference, generally, a xcex3 correction table used to convert luminance data into density data is prepared for each of image reading using the ADF and reading an original on the original table. By preparing different data, the input image densities are corrected to be substantially equal. In this case, however, the dynamic range of gradation of the input image becomes narrow.
In addition, in the conventional image reading unit, the storage time of photoelectric conversion elements does not change between generation of shading correction data and actual read of an original image independently of the elapsed time from the start of turning on the light source, as described above.
For this reason, when a light source of a certain type with an unstable light amount, i.e., a light source whose light amount is small at the start of turning on, increases then, and gradually decreases again is employed in the image reading unit, and a plurality of originals are to be continuously read, the image cannot be read in accordance with the change in amount of light emitted from the light source. That is, the light amount of the light source in reading the first original, that in reading several originals, and that in reading the final original are different. As a result, the input images corresponding to the plurality of originals have a density difference.
The present invention has been made to solve above described problems. It is an object of the present invention to provide an image reading apparatus for generating a satisfactory input image, when an original image is read, in any reading mode of a plurality of reading modes.
In order to achieve the above object, an image reading apparatus according to the present invention is characterized by comprising the following arrangement.
An image reading apparatus for reading an original image using a line sensor having a plurality of photoelectric conversion elements, characterized by comprising:
a memory for storing a charge storage time of the photoelectric conversion elements in correspondence with each of first and second reading modes;
detection means for detecting the first or second reading mode to be used to read the original image; and
image reading means for reading out a charge storage time corresponding to the reading mode detected by the detection means from the memory and reading the original image with the line sensor in accordance with the selected charge storage time.
The memory preferably stores a charge storage time for reading a reference portion and a charge storage time for reading an original image using data (shading correction data) generated upon reading the reference portion in correspondence with each of the first and second reading modes.
In the apparatus arrangement, the image reading means is of a type that reads
in the first reading mode, an image of an original placed on an original table while moving the line sensor, and
in the second reading mode, an image of a fed original at a predetermined position while fixing the line sensor at the predetermined position.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.